It is well known that zinc can be recovered from zinc bearing sulphidic concentrates by reacting the sulphides with sulphuric acid in the presence of oxygen at elevated temperatures and pressures. At temperatures above the melting point of sulphur, the elemental sulphur which is formed in the leaching reactions is present as finely divided liquid sulphur globules. As the reaction proceeds, the quantity of these globules increases and they coat or occlude unreacted sulphide particles, rendering them inert to further oxidation. Additives can be added to the process which prevent, or at least substantially inhibit, molten sulphur from coating the unleached sulphide particles, thereby allowing the reactions to proceed until high extractions of zinc, typically in excess of 95% and preferably in excess of 98%, are achieved, as taught for example, in U.S. Pat. Nos. 3,867,268 and 4,004,991. Further, the additives assist in the formation of a finely divided leach residue, with good physical handling characteristics. Without the additives, zinc extraction is typically only about 50-70%, and the liquid elemental sulphur may agglomerate, leading to the production of coarse, difficult to handle particles, which plug pipelines and vessels.
The additives used in this process must be compatible with the oxidation of zinc sulphide, and must not introduce impurities into the zinc bearing process streams. A number of surface active agents have been used for this purpose, including organic compounds such as lignin derivatives, particularly calcium and sodium lignosulphonates; tannin compounds, particularly tree bark and heartwood extracts such as quebracho, hemlock and redwood extracts; orthophenylene diamine; and alkaryl sulphonates, particularly sodium alkylbenzene sulphonates. Calcium lignosulphonate and quebracho have been applied in commercial practice. The current state of the art relating to the use of soluble surface active agents in zinc pressure leaching is summarized in a recent paper entitled "Effect of Surfactants on Zinc and Iron Dissolution during Oxidative Leaching of Sphalerite" by G. Owusu et at., Hydrometallurgy 38 (1995) 315-324.
The majority of these prior art additives are soluble in the acid zinc sulphate solution, and rapidly decompose under the high temperature highly oxidizing conditions of the zinc pressure leach process. Consequently at times it may be difficult to ensure that sufficient surfactant will always be available in the pressure vessel to prevent the occlusion of unleached sulphide particles by molten sulphur. Furthermore, the additives are usually added to make-up water prior to introduction to the autoclaves as a solution which exhibits a tendency to froth and overflow thereby causing potential loss of additive. In extreme cases, if the additive-containing solution fails to contact the zinc sulphide/sulphuric acid slurry, the absence of surfactant can result in the agglomeration of large amounts of molten sulphur into a separate heavy liquid phase, which causes plugging of pipelines and reaction vessels. Furthermore, the prior art surfactants are relatively more expensive, and in the case of the tree bark and heartwood extracts are in limited supply. It will be appreciated therefore that there exists a need for additives functional to prevent the occlusion of sulphide particles by elemental sulphur, which are both inexpensive, readily available and which are not rapidly decomposed under the conditions of the zinc pressure leach process.